1. Field of the Invention
The present invention relates to the demodulating of OFDM (Orthogonal Frequency Division Multiplexing) digitally modulated signals, and in particular, relates to an equalizer and an equalization method using pilot symbols.
2. Description of the Related Art
Terrestrial Integrated Services Digital Broadcasting (ISDB-T) employs OFDM digital modulation which is strong against multipath interference. In demodulating a signal which has been modulated by OFDM modulation (hereinafter, “OFDM signal”), a scattered pilot method which disperses pilot symbols in the frequency direction and the time direction is used as the amplitude phase standard for demodulation.
Japanese Patent Application Laid-Open (JP-A) No. 2000-22661 discloses an OFDM demodulator, and in particular, an automatic equalizer which carries out inverse Fourier transformation for equalization processing which removes distortion of the propagation path. A conventional OFDM demodulator receives a signal having a frame structure formed from frame symbols, a pattern signal for propagation path estimation which is continuous with the frame symbols, and data symbols which are continuous with the pattern signal for propagation path estimation. The conventional OFDM demodulator estimates a transfer function by using the pattern signal for propagation path estimation, and carries out equalization of the data symbols by using the estimated transfer function.
However, because the OFDM demodulator of JP-A No. 2000-22661 presupposes reception of a signal having the above-described frame structure, the OFDM demodulator has the problem that it cannot demodulate a signal of a data structure in which pilot symbols are scattered among data symbols such as used in Terrestrial Integrated Services Digital Broadcasting.
Further, an IFFF circuit 5-1 of an OFDM signal receiver of JP-A No. 2005-45664 inverse fast Fourier transforms scattered pilot signals of the past four symbols including the latest one symbol (paragraph [0015]). However, in the OFDM signal receiver of JP-A No. 2005-45664, the signal after the inverse fast Fourier transform is merely passed through an LPF, and the noise component included in the region of passage at the LPF cannot be removed in advance. Therefore, the OFDM signal receiver of JP-A No. 2005-45664 has the problem that, as the noise included in the received signal increases, the error in the results of estimating the transfer path becomes greater and the receiving characteristic deteriorates.
Further, an OFDM signal equalizer of JP-A No. 2004-153811 discloses a 4-symbol delay section (see FIG. 5). However, JP-A No. 2004-153811 does not utilize inverse Fourier transformation, but relatively compares the pilot symbols of the same sub-carrier existing four symbols before. Therefore, changes in the transfer function of the transfer path cannot be known until the pilot symbols of the same sub-carrier appearing four symbols after emerge. There is the problem that, if the transfer function of the transfer path changes within four symbols, it is easy for a case to arise in which equalization cannot be carried out correctly.